alternator-test: clean up conftest.py
alternator-test: clean up conftest.py

Remove unused random-string code from conftest.py, and also add a
TODO comment how we should speed up filled_test_table fixture by
using a batch write - when that becomes available in Alternator.
(right now this fixture takes almost 4 seconds to prepare on a local
Alternator, and a whopping 3 minutes (!) to prepare on DynamoDB).

Signed-off-by: Nadav Har'El <625169824b719df98706d88646d0aae6f368ae4d@scylladb.com>

scylladb/scylla,avikivity/scylla,scylladb/scylla,scylladb/scylla,avikivity/scylla,scylladb/scylla,avikivity/scylla